The present invention relates to a liquid crystal display device and, more particularly, to a transmissive type twisted-nematic liquid crystal display device which remarkably improves display quality in a black and white image display device or a multi-color image display device.
In recent years, transmissive type twisted-nematic liquid crystal display devices employing three primary color (red, green, and blue) filters have been developed for displaying a multi-color image. To produce a color television using the twisted-nematic liquid crystal display device, driving methods, the construction of the primary color filters, and liquid crystal materials to be employed have been studied. The problem of the above display device is how to obtain clear color and a wide hue. In the display device which is driven by a multiplex driving method, the above problem has not been discussed.
In the display device having the three primary color filters, each single-color filter in each combination of the three primary color (red, green, and blue) filters is disposed at each of picture elements corresponding to the intersections between the X and the Y electrodes arranged in a matrix, and the three primary colors disposed on each of the picture elements are mixed with each other by using an optical shutter effect of liquid crystal in a twisted-nematic mode.
Accordingly, it is required that display characteristics in the twisted-nematic mode on the picture elements must be common or uniform independent of a display portion on the display surface and an observing angle of an observer.
FIG. 1(A) shows a sectional view of a liquid crystal display cell for explaining a operation principle for a multi-color display.
A pair of glass substrates 2 and 2' interposing a liquid crystal material layer 20 is disposed between a pair of polarizers 1 and 1', and transparent electrodes 3 and 3' are formed on the inner surfaces of the glass substrates 2 and 2', respectively. Picture elements are disposed at the intersections between the electrodes 3 and 3' arranged in a matrix. The polarizer 1' functions as an analyzer.
Each single-color filter of red-color filters 4, green-color filters 5, and blue color filters 6 is scattered on each of the transparent electrodes 3' corresponding to the picture elements, respectively.
The liquid crystal material layer 20 contains liquid crystal molecules 7 which are oriented in a twisted-nematic mode.
An alternating voltage is applied to each of the electrodes 3 and 3' from a power source 9 by switching each of switches 8a, 8b, and 8c corresponding to each of the color filters 4, 5, and 6. The orientated direction of liquid crystal molecules interposed in each intersection between the electrodes 3 and 3' changes in response to the alternating voltage.
A light source 10 is arranged behind the liquid crystal display panel. When light from the light source 10 passes through the display panel and the alternating voltage is applied to each of the electrodes 3 and 3' corresponding to the respective color filter through the switch 8a, 8b or 8c, a multi-color can be displayed on the display surface of the display panel.
FIG. 1(B) shows a relationship between a wavelength of light and a light transmittance when each of the switches 8a, 8b, and 8c is turned on and off.
When the switch 8a, 8b, or 8c is turned on, the light having a hue corresponding to one of the primary color filters 4, 5, and 6 relating to the switch turned on has a maximum light transmittance so as to provide multi-colors.
Because the electro-optical characteristics of the liquid crystal panel in the twisted-nematic mode is dependent on an observing angle .theta. as shown in FIG. 2, in case where the display panel is observed by the observor 11 confronted with the display surface of the display panel as shown in FIG. 3, in which he notices that display characteristics differ between two picture elements disposed in the vertical direction on the display surface of the display cell or two picture elements disposed in the horizontal direction on the display surface. An angle between the two picture elements disposed in the vertical direction is designated by .DELTA..epsilon. around the observer 11, and an angle between the two picture elements disposed in the horizontal direction is designated by .DELTA..chi. around the observer 11. The difference of the display characteristics causes color diffusion and the reduction of a color contrast.
In FIG. 2(A), the liquid crystal molecules close to the glass substrate 2 are oriented in the direction of a vector r.sub.1, while the liquid crystal molecules close to the glass substrate 2' are orientated in the direction of a vector r.sub.2. The polarizers 1 and 1' have directions of polarization expressed by vectors P and A, respectively.
The X-, Y-, and Z- coordinates are defined on the display surface of the display panel. The incident direction of light is designated by a vector B.sub.1. The observing direction of the observor 11 is designated by a vector B.sub.2. An angle .theta. represents an angle formed between the observing direction and the Z axis. An angle .phi. represents an angle formed between the X axis and a projection of the observing direction vector B.sub.2 on an XY coordinate plane.
FIG. 2(B) shows characteristics between a supply voltage and the light transmittance on the angle .theta., for example, .theta.=0, 15, 30, 45 degrees, when the angle .phi.is 45 degrees. The light transmittance is dependent on the observing angle. For example, as the angle .theta. becomes greater, a high light-transmittance can be shown by supplying a low voltage.
FIG. 4 shows a sectional view of the conventional transmissive type twisted-nematic display device construction. The twisted-nematic liquid crystal display device comprises a liquid crystal display panel D, a diffuser 12, and a light source 10, and a case member.
The liquid crystal display panel D, and the diffuser 12, and the light source 10 are supported in the case member. The light source 10 such as a fluorecent light source for emiting white color light is disposed behind the display panel.
The diffuser 12 scatters light emitted from the light source 10, and applies the scattered light to the display surface of the display panel D through the liquid crystal material layer.
Because the light in the various directions is supplied to each of the picture elements in the liquid crystal display panel D by scattering the light from the light source 10, the utility factor of incident light becomes low, so that the contrast value may be remarkably reduced to less than the possible maximum contrast value. Thus, in the conventional display device, the reduction of the color contrast cannot be avoided, and it may be difficult to obtain a high quality image or a multi-color image on the display surface of the liquid crystal display panel.